This invention relates to coated films, and, in particular, to an apparatus and method for producing a coated tubular film.
Films are coated for a variety of reasons, for example--to improve gas-barrier characteristics or to confer heat-sealing properties on a surface of a relatively inert film. Such coatings are usually applied by depositing a coating material in flowable condition, for example--as a melt, or as a solution or dispersion of the coating material in a liquid vehicle, on to a surface of the film, and spreading the deposited coating material over that surface by means of an appropriately positioned cooperating spreading member. The deposited coating may be spread directly on the surface of a tubular film-forming extrudate by a stationary annular spreading member, suitably in the shape of a closely fitting, abrasive-filled, resilient collar encircling the extrudate. Alternatively, a tubular film may first be collapsed to the lay-flat state, and the deposited coating spread by means of at least one flat member, such as a bar or roller, engaging each of the opposed surfaces of the collapsed tube. The hitherto practised techniques thus generally involve longitudinal axial movement of the tubular substrate relative to the spreading member, so that the deposited coating material is spread longitudinally on a surface thereof, and, in practice, have proved difficult to operate to yield consistently uniform, high quality coatings.
A recently developed technique for coating an axially moving tubular form involves depositing a coating medium on a surface of the form, and spreading the deposited medium around at least part of the surface in a direction transverse to the direction of movement of the form. Transverse spreading is effected by means of an endless belt cooperating with the external surface of the form. However, a single belt assembly can effectively spread the coating medium around only part of the form surface, and complete coverage of the form surface entails the use of a plurality of independently activated belts axially spaced-apart along the length of the form.
We have now devised a simplified and reliable transverse coating system capable of yielding consistently uniform coatings.
Accordingly, the present invention provides an apparatus for producing a coated film in which a cooled thermoplastic polymeric tubular extrudate is axially forwarded through reheating and inflating means to yield an oriented, coated tubular film characterised by a coating head disposed prior to said reheating means and comprising a rotatable assembly including a support member providing an orifice through which the tubular extrudate may be fed in an axial direction, a plurality of spreading members spaced-apart around the orifice, mounting means for each of the spreading members, said mounting means being operatively associated with the support member and displaceable relative thereto, and means for biasing each of the spreading members inwardly of the support member for engagement with the external surface of the tubular extrudate, and by drive means for rotating the assembly in a plane substantially normal to said axial direction.
The invention further provides a method of producing a coated film by depositing a coating medium on the surface of an axially moving cooled thermoplastic polymeric tubular extrudate and reheating and inflating the coated extrudate to form an oriented, coated tubular film characterised by polishing the deposited coating medium onto the surface of the extrudate by successive contact with each of a plurality of spreading members resiliently biased into engagement with said surface and rotating around the periphery of the extrudate in a plane substantially normal to the longitudinal axis of said tubular extrudate.
Although tubular extrudates of various cross-sectional shapes may be coated, the technique of the present invention is particularly suitable for the coating of a substantially cylindrical extrudate from which an oriented film can be produced on inflation and stretching by a conventional "bubble" film-forming process.
Reference in this specification to movement of the tubular extrudate in an "axial" direction, unless otherwise stated, indicates movement substantially in the direction of the longitudinal axis of the extrudate, and includes movement in a direction slightly inclined to said longitudinal axis, to accommodate the sway or wander normally experienced in conventional processes for the manufacture of tubular films. Movement of the extrudate through the coating apparatus may be effected under the influence of gravity, or by conventional moving means, such as cooperating rollers, or endless belts, which engage the external surface of the extrudate to forward the latter in the desired direction.
Suitably, the support member of the coating apparatus is of any configuration defining an appropriate orifice through which the tubular extrudate may be passed in an axial direction, and conveniently is an annular member or plate containing a circular orifice of diameter slightly greater than that of the extrudate so that the latter may pass freely through the orifice without abrading against the edge thereof. The support member may be of unitary construction or may comprise a plurality of support elements arranged to provide a relatively rigid support structure or framework which serves as a base on which the respective spreading members may be mounted.
The function of the spreading members is to smear and spread an applied coating medium around the surface of the extrudate in a direction transverse to the axial direction of movement thereof, and it is preferred that the spreading members are disposed at locations regularly spaced-apart around the orifice in the support member, and thus around the periphery of the tubular extrudate. The number of spreading members employed is not critical provided there are sufficient thereof to ensure spreading of the applied medium into a continuous coating around the entire periphery of the extrudate. The number employed depends, inter alia, upon the dimensions of the tubular extrudate, and, in practice, is conveniently from 2 to 20, preferably from 6 to 12.
Each spreading member, which conveniently is of generally rectangular shape, includes a contact surface for presentation to the external surface of the tubular extrudate, and that contact surface may be planar or of appropriate curvature to conform closely to the surface of the extrudate.
Effective spreading of the applied coating medium is achieved if each spreading member assembly is of low inertia, and if each member per se is of relatively high flexibility thereby permitting the member to "track", and respond rapidly to, variations in the contour of the tubular extrudate. Preferably, therefore, each spreading member is fabricated from a resilient material of relatively low mass, such as spring steel. Radially inward flexing of a spreading member, relative to the longitudinal axis of the extrudate, may be assisted by ensuring that at least one of the circumferentially spaced-apart edges of that member is unsupported and capable of relatively unrestricted movement in a plane normal to said longitudinal axis. Conveniently, therefore, only the leading edge of each spreading member, in the direction of rotation of the assembly, is supported. Tracking of the extrudate contours may be further improved, if desired, by providing at least one, and preferably both, of the axially spaced-apart edges of a spreading member with a "neck" or indentation, preferably adjacent the supported edge of the spreading member, thereby permitting that spreading member to adopt an axially inclined position relative to the longitudinal axis of the extrudate. Axial inclination of a spreading member is particularly desirable when coating an extrudate in a transitional state between a partially collapsed and fully inflated condition--as encountered, for example, during the forwarding, by cooperating nip rolls, of a cast tubular extrudate for reheating and subsequent conversion to an oriented tubular film.
The contact surface of each spreading member presented to the tubular extrudate is suitably provided with a contact pad for direct engagement with the surface thereof, the pad being of a material capable of spreading and polishing the deposited coating medium to an acceptably uniform finish. The pad material is desirably soft, and inert to the coating medium, and should be selected so as to exhibit an acceptable life span despite the conditions encountered during the coating operation; e.g. the pad may have to operate in a relatively high temperature environment, and be subjected to a constant abrading action against the surface of the tubular extrudate. A resilient foam, such as a polyurethane foam, may be employed as the contact pad. Preferably, however, the contact pad should be of a material which does not absorb the coating medium and therefore retains its resilience and consistency when wetted by the coating medium; neither should it have a porous structure such that the coating medium will dry out to form a hard crust within the pores of the contact pad. Expanded elastomers having closed cell structures overcome this difficulty. A particularly suitable contact pad comprises a neoprene foam backing layer having adhesively bonded thereto, for engagement with the tubular extrudate, a layer of a nylon fabric.
The contact pad may be secured to the contact surface of the spreading member in any convenient manner. For example, the pad may be adhesively bonded to the contact surface, but is preferably detachably secured thereto--for example, by stud fastenings, or the like, positioned at the radially outward surface of the contact member to avoid abrasion of the extrudate surface.
The mounting means for each spreading member should be such that the latter is displaceable in a generally radially inward direction towards the axis of the extrudate and is retractable therefrom when coating of the surface of the extrudate is not required. Any suitable form of radial displacement mechanism, including a slideable assembly, may be employed, but a particularly simple and effective mounting means comprises a longitudinal mounting member pivotally attached about an intermediate point to the (annular) support member for pivotal movement in a plane substantially normal to the longitudinal axis of the tubular extrudate. An axially projecting finger or pin at or adjacent one end of each longitudinal mounting member provides a convenient support to which a spreading member may be secured--preferably detachably, to facilitate replacement thereof.
As hitherto described, better "tracking" by the spreading member and less "lift-off" of the latter from the extrudate surface is experienced if the spreading assembly is of low inertia. Preferably, therefore, the mounting member and associated components are fabricated from a material exhibiting strength and low mass, such as an aluminium alloy.
Biasing of each spreading member into an operative position for engagement with the extrudate surface may be effected in any convenient manner. For example, the aforementioned pivotal mounting member may be provided with an appropriate mass at or adjacent the end remote from the axial finger supporting the spreading member, whereby, on rotation of the entire spreading assembly, the centrifugal force acting on the mass causes sufficient pivotal movement of the mounting member to displace the spreading member to the operative position. Alternatively, and preferably, each mounting member may be resiliently biased to the operative position by suitably positioned spring means, compression or tension as appropriate, mounted on a spatially fixed base--such as the (annular) support member.
Desirably, the coating assembly includes means for retracting each spreading member from an operative position to an inoperative position in which the contact pad does not engage the extrudate surface. Independent retraction of individual spreading members may be practised, if desired, but for convenience and ease of operation it is preferred that retraction of all the spreading members is effected simultaneously. A particularly suitable device for effecting simultaneous retraction comprises an iris diaphragm positioned relative to the support member to cooperate with the respective mounting members and displace the latter against the force exerted by the biasing means, thereby disengaging the spreading members from the operative position.
The driving means for rotating the coating assembly is conveniently of a conventional kind, and may include a belt and pulley drive, or a gear wheel and pinion mechanism, coupled to a suitable prime mover, such as an electric motor.
The speed of rotation of the coating assembly in a plane substantially normal to the longitudinal axis may vary over a considerable range, the correct speed for acceptable coating performance being readily determined by simple experimentation. Conveniently, the ratio of rotational speed of the coating assembly to the axial speed of the tubular extrudate is within a range of from 2:1 to 20:1, and preferably from 5:1 to 10:1. Thus, a tubular film-forming extrudate travelling in an axial direction at a speed of some 100 mm/s is conveniently coated by a device according to the present invention rotating at a speed of the order of 700 mm/s, i.e. a ratio of 7:1.
Deposition of the coating medium onto the external surface of the tubular extrudate is effected in any convenient manner--for example, by brushing, by discharge from a suitably positioned manifold, by pumping in the form of an aerated foam, or by trickle or drip feed directly onto the contact pads of the spreading assembly. However, to avoid profile defects, e.g. in film derived from the coated extrudate, it is preferred that the coating medium is applied by a technique which avoids "point-contact" (as with fixed applicator brushes) of the medium on the extrudate surface and affords good preliminary spreading of the medium prior to contact with the spreading members. Accordingly, the coating medium is conveniently applied from a spray assembly upstream of the spreading assembly, in the direction of travel of the tubular extrudate. The spray assembly suitably comprises a plurality (e.g. four) of spray heads regularly disposed around the periphery of the tubular extrudate, these heads preferably being independently adjustable with respect to direction and velocity of delivery.
In one embodiment of the invention a recovery vessel for collecting excess coating medium is provided, suitably in the form of an annular receptacle or drip-tray located downstream of the spreading assembly. Recycling of the collected medium may be effected, if desired.
Centralising means for accurately positioning the coating assembly relative to the path of the tubular extrudate may also be provided--suitably by means of a plurality of threaded adjusters appropriately located around the periphery of the assembly.
Although, in general, coatings in flowable condition, for example--emulsions, slurries, pastes, gels and foams, are suitable for application by the technique of the present invention, we prefer, for convenience and ease of application, to employ coatings in the form of a solution or dispersion of the coating material in a liquid medium. An aqueous solution or dispersion of the coating material is particularly convenient in terms both of cost of the liquid medium, and of safety in avoiding the explosive and toxicity hazards attendant upon the use of volatile organic solvents.
The transverse spreading technique of the present invention ensures that the applied coating medium is positively spread and smoothed by the polishing action created by the tangential wiping motion of the spreading members relative to the surface of the moving extrudate. This assists the formation of a uniform coating, and avoids "flooding" of the coating medium at regions of the extrudate exhibiting surface indentations and irregularities.
To improve the spreadability of the coating medium and ensure that it remains uniformly dispersed in a continuous layer after spreading on the extrudate surface, properties such as the viscosity and surface tension of the coating medium can be adjusted to a suitable value--for example by inclusion in the coating medium of a conventional viscosity modifier, such as a water-soluble polymer, and/or an appropriate surfactant. The appropriate balance of these characteristics depends, inter alia, on the temperature of the coating environment and the rate at which the applied coating is allowed to dry, and is readily established by simple experimentation. Thus, a relatively viscous coating medium, for example--a medium having a viscosity (measured with a Brookfield Viscometer, spindle No. 6) of the order of 22,500 centipoise (10 rpm) or 5,350 centipoise (100 rpm), can be employed.
The technique of the present invention may be employed in the application of coatings of various kinds, but has proved particularly useful in the production of thermoplastic polymeric films having an electrically continuous antistatic coating on a surface thereof. For example, a coating medium comprising an aqueous solution of a quaternary ammonium compound, such as choline chloride, as antistatic agent may be employed. An amine sulphate prepared from `Ethomeen` T12 (supplied by Armour Hess Chemicals Limited) has also proved of value as an independent antistatic agent, and additionally exhibits surfactant characteristics. Accordingly a combination of choline chloride and `Ethomeen` T12 sulphate in aqueous solution has proved to be a particularly effective antistatic coating medium for use according to the present invention. Other antistatic agents, alone or in combination, may be employed, if desired.
The amount of amine sulphate present as the sole antistatic agent in the solution or dispersion applied to the tubular extrudate depends, inter alia, on the level of antistatic properties required in the treated product, and can be readily determined by simple experimentation. Relatively high concentrations of amine sulphate are suitable, provided that the viscosity of the solution or dispersion is not increased to a level which adversely affects the mobility and spreadability thereof, and are advantageous in that the amount of volatile vehicle, which may adversely affect the surface of the extrudate during evaporation therefrom, is kept to a minimum. In practice, the amine sulphate is conveniently employed at a concentration of up to about 60% by weight of the solution or dispersion, and is suitably employed at concentrations within a range of from 5 to 55%, preferably from 10 to 50%, by weight of the solution or dispersion. However, if the antistatic influence of the amine sulphate is supplemented by the presence of an additional additive, such as choline chloride, the concentration of the amine sulphate may be reduced to a relatively low value, for example of the order of 0.1 to 2.5%, the total concentration of the amine sulphate and the additional additive being within the aforementioned range of up to 60% by weight of the solution or dispersion. Suitably, the concentration of the amine sulphate, alone or together with a supplementary additive, is selected so as to yield a product, such as a polyolefin film derived from the coated extrudate, exhibiting a surface resistivity, measured at 50% Relative Humidity, not exceeding 10 gigohms, and preferably less than 5.0 gigohms.
In practice, we have observed that adequate surfactant behaviour is achieved by the use of a relatively small amount of a long chain amine sulphate. Such salts are less effective, weight for weight, as antistatic agents than short chain quaternary ammonium compounds, such as choline chloride, and we therefore prefer to employ a solution or dispersion comprising a major proportion of the quaternary ammonium compound and a minor proportion of the amine sulphate. Conveniently, the weight ratio of quaternary ammonium compound to amine sulphate in the solution or dispersion applied to the tubular extrudate is from 2:1 to 50:1, preferably between 3:1 and 30:1. As hereinbefore described, the combined concentration of quaternary ammonium compound and amine sulphate is desirably such, for example up to about 60% by weight of the applied solution or dispersion, that the viscosity of the solution or dispersion is not increased beyond a level at which a uniform distribution of the additives on the substrate can be achieved.
Another effective antistatic composition comprises a quaternary ammonium compound, such as choline chloride, an organic polyol, such as glycerol, and, optionally, an amine salt of the kind hereinbefore described. The composition conveniently comprises from 10 to 90 weight % of the quaternary ammonium compound, from 90 to 10 weight % of the organic polyol, and from 0 to 50 weight % of the amine salt, and may be employed at a concentration of from 1 to 50 weight % of an aqueous solution.
The amount of coating medium employed will depend, inter alia, on the application envisaged for the coated film, and on the required characteristics of the coated surface--such as coefficient of friction and/or electrical conductivity, but, in the case of an oriented polyolefin film substrate the coating medium is conveniently applied in an amount which will yield an average dry coat thickness within a range of from 0.0005 to 0.03 .mu.m, preferably from 0.001 to 0.002 .mu.m.
To achieve adequate spreading of the applied coating medium there should be relative transverse movement between the spreading members and axially moving tubular extrudate. In practice, this is conveniently achieved by ensuring that the tubular extrudate is non-rotatable about its longitudinal axis, in the direction of movement of the spreading members, to any significant extent during the spreading operation.
After application, and spreading, the applied coating medium may be formed into a continuous dry, adherent, coating layer on the tubular extrudate by conventional procedures--for example, by allowing the coating to dry at ambient temperature, or by passing the coated extrudate through a drying oven to evaporate the solvent or dispersant medium.
The present invention is particularly suited to the production of self-supporting coated films formed from a thermoplastic film-forming polymeric material.
By a "self-supporting" film is meant a film capable of independent existence in the absence of a supporting substrate, a polyolefin packaging film being a typical example thereof. Suitable thermoplastic film-forming polymeric materials include polycarbonates, polysulphones, polyamides such as polyhexamethylene adipamide or polycaprolactam, polyesters such as polyethylene terephthalate and polyethylene-1,2-diphenoxyethane-4,4'-dicarboxylate, vinyl polymers and copolymers, and polymers and copolymers of 1-olefins such as ethylene, propylene, butene-1,4-methylpentene-1. A preferred material is a high molecular weight stereoregular predominantly crystalline polymer of propylene, either in the form of a homopolymer or copolymerised with minor quantities (e.g. up to 15% by weight of the copolymer) of other unsaturated monomers, such as ethylene.
An oriented tubular film is suitably produced by melt extruding the desired polymeric material in tubular form from a simple annular die, cooling the extruded tube, reheating and inflating the tube by the so-called "bubble" process to introduce transverse orientation, and simultaneously elongating the tube longitudinally to orient the film in a lengthwise direction. The film is then preferably "heat-set", i.e. dimensional stability of the film is improved by heating the film, while restrained against thermal shrinkage, to a temperature above the glass transition temperature of the polymer from which the film is formed but below the melting point thereof.
A similar technique employing a multi-channel, annular, coextrusion die is suitable for the production of multiple-layer films, such as a polypropylene substrate having on at least one surface thereof a layer of a copolymer of propylene (80 to 95% by weight) with another alpha-olefin containing from 4 to 10 carbon atoms, such as butene-1.
In the production of a coated film according to the invention, the coating medium is conveniently deposited and spread on a surface of the cast, unoriented tubular extrudate immediately prior to the reheating and orienting stage of the film-forming process. Drying of the coating, for example--by evaporation of the volatile solvent or dispersant, is therefore effected during the reheating operation, and the dried coating layer becomes firmly bound to the film surface during orientation.
Coated films made according to the present invention may contain any of the additives conventionally employed in the production of thermoplastic films, and may be subjected to conventional after-treatments--for example, exposure to a corona discharge treatment to improve the bonding and print-receptive characteristics of the film surface.
Films made according to the present invention may vary in thickness depending on the intended application, but usually we find that films having a thickness of from 2 to 150 microns are of general utility. Films intended for use in packaging operations are suitably within a thickness range from 10 to 50 microns.